Plasma display panels (PDP), as used in high definition television (HDTV) and projection television (PTV) applications, conventionally use di-valent europium activated barium magnesium aluminate (BAM) phosphor as a blue emitting component due to its availability and high quantum efficiency. However, compared with other phosphors such as Eu.sup.3+ (red) and Tb.sup.3+ (green) activated yttrium, gadolinium borate based phosphors (BAM) exhibits a wide spectrum of emission with lower color purity and very short lifetime with the VUV flux. Therefore, efforts have been made to develop new phosphors to replace BAM and to provide improved performance characteristics. Lifetime of a plasma display is directly related to the performance of phosphors used in the display. Therefore, lifetime of phosphors is a great concern in selecting suitable phosphors. Displays should exhibit lifetimes of the order of 30,000 hours of operation.
Most of the work reported to date on lanthanum phosphate based phosphors has been related to fluorescent lamp applications as an efficient green phosphor and the performance of the phosphor therein. Development of terbium and cerium activated lanthanum phosphate is well documented in numerous patents. Different methods of preparation and the introduction of various impurities have been tried in attempts to improve the life and performance of the lamp. U.S. Pat. No. 4,423,349 to Nakajima et. al. describe two methods of synthesizing the above phosphor. In the first method, lanthanide carbonates are reacted with phosphoric acid at 75.degree. C. and then calcinated at 1150.degree. C. for 75 minutes. In the second method, coprecipitated lanthanide oxalates are oxidized to a single phase lanthanide oxide at 800.degree. C. Diammonium phosphate is mixed with the oxide and fired at 1200.degree. C. Boron oxide or ammonium borate is also added before calcination to enhance the reaction and also improve the brightness.
U.S. Pat. No. 5,091,110 to Albert et. al. discloses a method of making lanthanum cerium terbium phosphate phosphor in a two step process. The method comprises formation of an aqueous solution of lanthanide nitrates and an aqueous solution of diammonium phosphate and combining both to coprecipitate a lanthanum terbium cerium phosphate followed by firing the mixture at higher temperatures to form the phosphor. Boron phosphate is used as the phosphate source because it is stable at elevated temperatures (see U.S. Pat. No. 5,132,042). Lithium carbonate is also used as a flux forming compound to improve the solubility of the lanthanide phosphate in the boron oxide solution formed during the process (see U.S. Pat. No. 5,154,852).
Terbium, cerium activated lanthanum phosphate is also prepared by reacting monoammonium phosphate solution and respective rare earth nitrate solutions (U.S. Pat. No. 5,340,556 to Collin et al.). The resultant powder is calcined at 900.degree. C. in air or in a non reducing atmosphere to obtain a phosphor with 250 nm compact aggregates. From XRD analysis, it is found that the resultant phosphor powder has monoclinic crystal structure. Small size phosphor particles could be prepared by adding excess boric acid and lithium carbonate as a flux in the starting mixture before firing (see U.S. Pat. No. 5,651,920 to Chau et al.).
U.S. Pat. No. 5,746,944 to Braconnier et al. disclose a lanthanun/cerium/terbium mixed green phosphor that is directly precipitated by reacting a first solution of soluble lanthanum, cerium and terbium salts with a second solution containing phosphate ions.
HDTV and similar type display devices should have high resolution and high brightness to meet expected performance. This can be achieved currently only with thin phosphor screens consisting of very small phosphor particles (0.5-2 microns) in a close rib structure. Screens with small particles have higher packing density and also need lesser binder content. It is known that terbium and cerium activated lanthanum phosphate have high quantum efficiency, better stability at operating temperatures and long lifetime, particularly under 254 nm UV excitation (compact fluorescent lamps). However, very limited information is available on the preparation and luminescent studies on thulium activated lanthanum phosphate phosphors.